package com.muyu.common.core.utils;

import java.net.InetAddress;
import java.net.UnknownHostException;

/**
 * ID生成器工具类
 * 使用雪花算法生成分布式ID
 */
public class IdGenerator {
    // 起始的时间戳（2023-01-01 00:00:00）
    private static final long START_TIMESTAMP = 1672502400000L;
    
    // 每部分占用的位数
    private static final long SEQUENCE_BIT = 12; // 序列号占用的位数
    private static final long MACHINE_BIT = 5;   // 机器标识占用的位数
    private static final long DATACENTER_BIT = 5;// 数据中心占用的位数

    // 每一部分的最大值
    private static final long MAX_DATACENTER_NUM = ~(-1L << DATACENTER_BIT);
    private static final long MAX_MACHINE_NUM = ~(-1L << MACHINE_BIT);
    private static final long MAX_SEQUENCE = ~(-1L << SEQUENCE_BIT);

    // 每一部分向左的位移
    private static final long MACHINE_LEFT = SEQUENCE_BIT;
    private static final long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
    private static final long TIMESTAMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;

    private long datacenterId;  // 数据中心
    private long machineId;     // 机器标识
    private long sequence = 0L; // 序列号
    private long lastTimestamp = -1L; // 上一次时间戳

    private static volatile IdGenerator instance;

    /**
     * 获取单例实例
     */
    public static IdGenerator getInstance() {
        if (instance == null) {
            synchronized (IdGenerator.class) {
                if (instance == null) {
                    try {
                        // 使用本机IP地址的最后一段作为机器ID
                        String hostAddress = InetAddress.getLocalHost().getHostAddress();
                        long machineId = Long.parseLong(hostAddress.split("\\.")[3]) % 32;
                        instance = new IdGenerator(1, machineId);
                    } catch (UnknownHostException e) {
                        // 如果获取本机IP失败，则使用随机数作为机器ID
                        instance = new IdGenerator(1, (long) (Math.random() * 32));
                    }
                }
            }
        }
        return instance;
    }

    private IdGenerator(long datacenterId, long machineId) {
        if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
            throw new IllegalArgumentException("datacenterId can't be greater than " + MAX_DATACENTER_NUM + " or less than 0");
        }
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException("machineId can't be greater than " + MAX_MACHINE_NUM + " or less than 0");
        }
        this.datacenterId = datacenterId;
        this.machineId = machineId;
    }

    /**
     * 生成下一个ID
     */
    public synchronized long nextId() {
        long currTimestamp = getNewTimestamp();
        
        if (currTimestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards. Refusing to generate id");
        }

        if (currTimestamp == lastTimestamp) {
            // 相同毫秒内，序列号自增
            sequence = (sequence + 1) & MAX_SEQUENCE;
            // 同一毫秒的序列数已经达到最大
            if (sequence == 0L) {
                currTimestamp = getNextMill();
            }
        } else {
            // 不同毫秒内，序列号置为0
            sequence = 0L;
        }

        lastTimestamp = currTimestamp;

        return (currTimestamp - START_TIMESTAMP) << TIMESTAMP_LEFT // 时间戳部分
                | datacenterId << DATACENTER_LEFT      // 数据中心部分
                | machineId << MACHINE_LEFT            // 机器标识部分
                | sequence;                            // 序列号部分
    }

    /**
     * 生成字符串形式的ID
     */
    public static String generateId() {
        return String.valueOf(getInstance().nextId());
    }

    private long getNextMill() {
        long mill = getNewTimestamp();
        while (mill <= lastTimestamp) {
            mill = getNewTimestamp();
        }
        return mill;
    }

    private long getNewTimestamp() {
        return System.currentTimeMillis();
    }
}
